1. Field of the Invention
The present invention relates to a viewfinder optical system. More particularly, though not exclusively, the present invention can be applicable to an image pickup apparatus such as a single-lens reflex camera.
2. Description of the Related Art
A single-lens reflex camera includes a photographic lens that forms a viewfinder image (i.e., an object image) on a focusing screen and a viewfinder optical system that enables a user to observe the image. The viewfinder optical system includes an image inverting unit (e.g., a pentagonal prism) configured to convert a viewfinder image formed on a focusing screen into an erected image. Furthermore, the viewfinder optical system includes an eyepiece lens configured to enlarge the erected image.
Typically it is desirable in the market that an eyepiece lens of a viewfinder optical system has a high observation magnification (or viewfinder magnification), a sufficiently long eye relief, a diopter adjusting function, and higher optical performances.
In general, the observation magnification of a viewfinder optical system can be defined as the ratio of the focal length of a photographic lens to the focal length of an eyepiece lens. For example, as the focal length of an eyepiece lens is short, the observation magnification is large. However, a viewfinder optical system for a single-lens reflex camera has a diopter fixed to a standard diopter (=approximately −1 diopter). Therefore, if the distance between the focusing screen and the eyepiece lens (more specifically, an optical path length to the principal point position of the eyepiece lens) is determined, the focal length of the eyepiece lens is substantially determined.
Hence, if a pentagonal prism has a short optical path length to shorten the distance between the focusing screen and the eyepiece lens, the eye-point of the viewfinder optical system shifts toward the object side compared to the rear surface of the camera. Thus, the eye relief becomes shorter. The eye relief is defined by a distance between the exit surface of the eyepiece lens and a pupil (eye-point) of an observer.
The pentagonal prism can be replaced with a pentagonal roof mirror, which is useful in light weight and low cost. However, a viewfinder optical system including a pentagonal roof mirror has a long air-equivalent optical path length compared to a viewfinder optical system including a pentagonal prism. Therefore, the use of a pentagonal roof mirror is not effective to enlarge the observation magnification.
As discussed in Japanese Patent Application Laid-open No. 2001-311881, an eyepiece lens for a viewfinder optical system including a pentagonal roof mirror can include three lens units, i.e., a negative lens, a positive lens, and a negative lens, which are disposed in order from the object side to the observation side. The viewfinder optical system allows a user to perform diopter movement by moving a part of the lens units along an optical axis.
As discussed in Japanese Patent Application Laid-open No. 2000-356799, a viewfinder optical system including a pentagonal roof mirror can obtain a higher observation magnification when a part of mirrors constituting the pentagonal roof mirror has a focal optical power.
Recent digital single-lens reflex cameras can be equipped with an image sensor comparable to the Advanced Photo System type-C (APS-C) size which is capable of providing an image circle smaller than that of 35 mm silver-halide film.
According to a single-lens reflex camera, a viewfinder image to be observed via a viewfinder optical system is equivalent to an object image formed on an imaging plane. Therefore, a viewfinder image in the single-lens reflex camera is smaller compared to a viewfinder image observed in a conventional camera for 35 mm silver-halide film.
A larger observation magnification can be obtained by increasing the number of lens elements constituting an eyepiece lens. However, the viewfinder optical system becomes complicated in structure.
Furthermore, the viewfinder magnification can be increased if a pentagonal roof mirror has a focal optical power. However, maintaining higher optical performances is required. Adequately correcting decentration aberrations is also required to perform diopter movement.